The present disclosure generally relates to conductive polymer compositions produced by a solvent exchange method, and more specifically to conductive polymer compositions produced by a method that involves the exchange of the water in a polythiophene dispersion with a specific mixture of organic solvents. The present disclosure also relates to methods for producing such compositions. This disclosure further pertains to the application of these compositions to fabricate a variety of articles, such as coatings, and to making and using the same in the fabrication of electronic and opto-electronic devices.
Conductive polymers (CPs) have received considerable attention in recent years due to their potential applications in a variety of electronic devices. The realization that organic polymeric materials could be made to exhibit electrical conductivity by doping was first discovered in 1977 [H. Shirakawa, E. J. Louis, A. G. MacDarimid, C. K. Chang and A. J. Heeger, J. Chem. Soc. Chem. Comm. 579 (1977)]. This discovery was considered such a breakthrough that the Nobel Prize in Chemistry was awarded to these researchers (McDiarmid, Heeger and Shirakawa) in 2000 for this work. CPs are presently used in commercial products as anti-static coatings on plastics such as photographic film and electronic packaging materials. Other applications include solid electrode capacitors, through-hole plating of printed circuit boards, coatings for cathode ray tubes (to prevent dust attraction), hole injecting layers on indium tin oxide (ITO) substrates for electroluminescent devices, and sensors. Future applications such as an ITO replacement leading to completely flexible, organic electronic devices will require improvement in conductivity without sacrificing other properties such as optical transparency.
A variety of conductive polymers have been prepared and characterized, and several are commercially available such as Baytron(copyright) P from Bayer and Panipol(copyright) from Uniax. Of the different CP families, [i.e. polyacetylenes, polyphenylenes, poly(p-phenylenevinylene)s, polypyrroles, polyanilines, and polythiophenes] polythiophenes are arguably the most stable-thermally and electronically [(xe2x80x9cHandbook of Oligio- and Polythiophenesxe2x80x9d, D. Fichou, Editor, Wiley-VCH, New York (1999), J. Roncali, Chem. Rev., 97, 173 (1997), A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem., 110, 416 (1998), J. Roncali, J. Mater. Chem., 9, 1875 (1999), J. Roncali, Annu. Rep. Prog. Chem. Sec. C., 95, 47 (1999), A. J. Heeger, Synth. Met., 55-57, 3471 (1993) and G. Kobmehl and G. Schopf, Adv. Polym. Sci., 129, 1 (1996)]. The Baytron(copyright) P product is a poly 3,4-ethylenedioxythiophene/polystyrene sulfonate (PEDOT/PSS) composition available as an aqueous dispersion containing xcx9c1.3% solids. This aqueous dispersion is typically used to prepare coatings on various substrates. Baytron(copyright) P coatings exhibit no change in conductivity after 1000 hours in air at 100xc2x0 C. and can survive intact at temperatures as high as 200xc2x0 C., albeit for shorter exposure periods. It is prepared from 3,4-ethylenedioxythiophene (EDT) in aqueous or predominately aqueous media in the presence of polystyrenesulfonic acid (PSS, dopant) using an oxidant such as iron trichloride [L. B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik and J. R. Reynolds, Adv. Mater., 12(7), 481 (2000)]. Coatings of Baytron(copyright) P have been reported to exhibit a wide range of surface resistance, depending upon thickness. It is well known for Baytron(copyright) P, as well as other CP based coatings, that the surface conductivity will increase with increasing coating thickness while the optical transmission will decrease. In most coating applications, the coatings must exhibit a specific combination of electrical conductivity, optical transparency and environmental stability (i.e. stability to moisture and oxygen) to be useful. The coating must exhibit good adhesion to the substrate as well. The appropriate balance or combination of these properties is of critical importance; thus, a means for improving this combination of properties would represent a significant advancement and enable new applications for these materials.
One approach to improve the electrical conductivity of polythiophenes is by the use of organic additives. It has been shown that certain additives, when mixed with Baytron(copyright) P aqueous dispersion and subsequently used to make a coatings, can produce an increase in the electrical conductivity (i.e. decrease in surface resistivity), however a high temperature treatment (xcx9c200xc2x0 C.) is also required [Jonas et al, U.S. Pat. No. 5,766,515, (1998) to Bayer AG]. The high temperature treatment is a major disadvantage since certain-substrates cannot tolerate this step. No explanation of the mechanism associated with conductivity enhancement is offered; thus, it is impossible to elucidate what additives may bring about this increase in electrical conductivity.
Another method has involved a solvent exchange process in which most or all of the water present in a Baytron(copyright)P aqueous dispersion is exchanged with an organic solvent (see U.S. Ser. No. 09/999,171; 60/298,174 and 60/269,606). Employing the solvent exchange method also brings about a fundamental change to the material that results in significant improvement in the combination of electrical conductivity, optical transparency environmental stability and adhesion characteristics to a variety of substrates. Consequently, this method enables the solvent exchanged product to meet specifications for a variety of applications that the aqueous based precursor cannot meet.
Surface resistance of CP based coatings is typically measured using a four-point probe device. Certain other measurements must also be performed, such as coating thickness, in order to calculate volume resistivity. The volume resistivity is calculated using the following equation:
Volume resistivity=(xcfx80/ln2)(k)(t)(surface resistance in ohm/square)
Wherein xe2x80x9ctxe2x80x9d is the coating thickness, measured in centimeters (cm), xe2x80x9ckxe2x80x9d is the geometrical correction factor, and xe2x80x9cln2xe2x80x9d is the natural log of 2. The constant k is related to the coating thickness, probe spacing and sample size. Due to the variables associated with these measurements, quantitative comparison between measurements of the volume resistivity of coatings performed using different devices and different operators can be problematic.
Organic polymers that are intrinsically conductive typically contain sp2 hybridized carbon atoms that have (or can be adapted to have) delocalized electrons for storing and communicating electronic charge. Some polymers are thought to have conductivities neighboring those traditional silicon-based and metallic conductors. These and other performance characteristics make such conductive polymers desirable for a wide range of applications. See Burroughes, J. H. et al. (1986) Nature 335:137; Sirringhaus, H. et al. (2000) Science, 290, 2123; Sirringhaus, H. et al. (1999) Nature 401: 2; and references cited therein, for example.
There is recognition that many conductive polymers can be used to coat a wide range of synthetic or natural articles such as those made from glass, plastic, wood and fibers to provide an electrostatic or anti-static coating. Typical coatings can be applied as sprays, powders and the like using recognized coating or printing processes.
However, there is increasing understanding that many prior conductive polymers are not useful for all intended applications. For example, many of such polymers are not sufficiently conductive or transparent for many applications. In particular, many suffer from unacceptable conductivity, poor stability, and difficult processing requirements. Other shortcomings have been reported. See e.g, the U.S. Pat. Nos. 6,084,040 and 6,083,635. Efforts have focused on improving properties of conductive polymers such as solubility or conductivity. However, for many applications, having an improvement in only one property, such as electrical conductivity, is not sufficient for the material to be useful as a coating or as a component in an electrical or opto-electronic device. Today""s applications for conductive polymers demand that the materials have specific combinations of properties that are often difficult to achieve in a single material. The properties of interest include, but are not limited to electrical conductivity, processing characteristics, optical transparency, adhesion to desired substrate, environmental stability, thermal stability and acceptable cost. Poly 3,4-ethylenedioxythiophene (commercially available as Baytron(copyright) P aqueous dispersion) has been reported to offer good conductivity, transparency, stability, hydrolysis resistance and processing characteristics. See Bayer AG product literature (Edition 10/97; Order No. A1 5593) Inorganics Business Group D-51368, Leverkusen, Germany. Other Baytron(copyright) formulations have been reported for use in specific applications. Illustrative formulations (P type) include CPUD2, CPP103T, CPP105T, CPP116.6, CPP134.18, CP135, CPP 4531 I, CPP 4531 E3 and CPG 130.6. Further information relating to using Baytron(copyright) formulations can be obtained from the Bayer Corporation, 100 Bayer Rd. Pittsburgh, Pa. 15205-9741. See also the Bayer Corporation website at bayerus.com the disclosure of which is incorporated by reference.
All commercially available and known Baytron(copyright) formulations are dispersions in aqueous or predominately aqueous media. In the patent literature relating to Baytron(copyright), the claims all pertain to an aqueous dispersions or predominately aqueous dispersions (for example see U.S. Pat. Nos. 5,300,575, 5,766,515 and 6,083,635. No PEDOT/PSS formulations have been disclosed in the art which are dispersed or solvated in a substantially organic solvent system, nor has a method of preparing such a formulation been previously disclosed. Thus, in all applications using or contemplating the use of Baytron(copyright) P prior to these disclosures, it was assumed that the desired article would have to be fabricated from a predominately aqueous based dispersion. There are many known applications wherein Baytron(copyright) P would be useful if not for the water present and if the combination of conductivity, transparency, adhesion and environmental stability could be improved.
Flexible electronic device xe2x80x9cwritingxe2x80x9d or xe2x80x9cprintingxe2x80x9d has attracted much recent attention. An example of such a technique involves dispersing an aqueous and conductive thiophene preparation with an ink-jet printer. Typically, poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT/PSS) is employed. See generally Dagni, R. in Chemistry and Engineering, Jan. 1, 2001, pp. 26-27 as well as references cited therein.
However, these writing or printing procedures have suffered for want of an effective and reproducible means of replacing water with one or more organic solvent(s).
There is recognition that many electrical devices such as thin film transistors (TFTs) and electro-optic devices, such as light emitting diodes (LED""s), particularly organic light emitting diodes (OLED""s), touch screen displays, smart windows, back lights for displays and photovoltaic cells, require substrates coated with an electrically conductive material that has high optical transparency, good adhesion to the substrate and ideally it can be applied in a one-step and continuous process. Typically these coated substrates function as electrodes, but they can perform other functions as well such as a hole injecting material. Coatings derived from Baytron(copyright) P aqueous dispersions have been tested in many of the aforementioned devices, however, significant technical problems were encountered, which could be traced back to water present in the dispersion of Baytron(copyright) P. Typically, the technical problems associated with prior Baytron(copyright) P coatings included unacceptable adhesion to the substrate, unacceptable environmental stability and insufficient optical transparency.
Presently, transparent electrodes in electro-optic devices are made of indium doped tin oxide (ITO) coated glass substrates. ITO/glass transparent electrodes are not flexible and the ITO coating is applied in a complex, expensive, batch oriented vacuum deposition process. ITO films are brittle and difficult to prepare and manipulate, particularly when used on plastics or large area substrates or flexible substrates. See generally Y. Cao, et al. in Conjugated Polymeric Materials: Opportunities in Electronics, Optoelectronics and Molecular Electronics, NATO Advanced Study Institute, Series E: Applied Sciences, J. L. Bredas and R. R. Chance, Eds., Vol. 82, Kluwer Academic, Holland (1990). See also U.S. Pat. No. 5,618,469 and EPO Pat. No. 686,662.
Thus, there is a need for a material that could replace ITO in some applications that could be processed in a continuous fashion. One such method is roll-to-roll coating of a continuous plastic web such as polyethylene terephthalate wherein the processing speeds could be dramatically increased and the cost of the product would decrease. Unfortunately, Baytron(copyright) P aqueous dispersions have failed to meet expectations in this application due to processing problems and unacceptable combination of conductivity, transparency, adhesion and environmental stability in the final coated plastic product.
Accordingly, there is a need for a method for producing organic solvent based conducting polymer compositions that offer an improved combination of properties such as processing characteristics, electrical conductivity, optical transparency, environmental stability and adhesion to a variety of substrates.
It is an objective of the present disclosure to provide a method for exchanging the water in an aqueous conductive polymer mixture with a specific mixture of organic solvents to produce organic solvent based conducting polymer compositions. Another objective of the present disclosure is to provide coatings from these compositions that, compared to coatings derived from the aqueous conductive polymer composition, exhibit a significant improvement in electrical conductivity, optical transparency, environmental stability and adhesion to glass and plastic substrates. Another objective of the present application is to apply the compositions and coated articles of the invention in applications and devices to provide improved performance.
The present disclosure relates to a solvent exchange method wherein water present in a commercially available Baytron(copyright) P aqueous dispersion is exchanged for a specific mixture of organic solvents. The resulting organic dispersion, comprising a conducting polymer dispersed in an organic solvent system, is dispersed in a mixture of organic solvents exhibits improvements in the following combination of properties, electrical conductivity, optical transparency, environmental stability, adhesion to a variety of substrates and processing characteristics. These conducting polymer compositions are useful for preparing coatings on a variety of substrates. They provide improved performance when used as coatings, layers or subcomponents in a variety of electronic and opto-electronic devices.